Alternating current arc power source



June 27, 1967 B. J. ALDENHOFF 3,328,637

ALTERNATING CURRENT ARC POWER SOURCE Filed Feb. 11, 1965 n42 I I YINVENTOR 7 Z. BERNARD .1 morn/arr United States Patent 3,328,637ALTERNATING CURRENT ARC POWER SOURCE Bernard J. Aldenhoif, oconomowoc,Wis., assignor, by mesne assignments, to Harnischfeger Corporation, WestMilwaukee, Wis, a corporation of Wisconsin Filed Feb. 11, 1965, Ser. No.431,826 11 Claims. (Cl. 315-277) ABSTRACT OF THE DISCLOSURE Theinvention involves an arc current supply system having a pair of surgestabilizing circuits. These circuits each have a pulse capacitor andtriggered switching means. Each surge stabilizing circuit facilitatesare reignition during welding. These circuits operate in c0njunctionwith an alternating current applied to the electrode and the workpiecebeing welded.

Welding arcs and similar high current arcs for melting and otherprocessing have been established with either alternating currents ordirect currents depending upon the particular work requirements. Sucharcs may employ a consumable electrode or a nonconsumable electrode.Further, the power source may be of the constant current variety havinga drooping voltage versus current characteristic or the constantpotential variety having a relatively flat characteristic. The constantpotential variety has been found to be preferred for many applications.

Alternating current power has certain advantages from simplicity ofbasic equipment. However, in alternating current arcs, the current ateach reversal or half cycle passes through zero and the arc must beactually reignited. This has introduced severe complications in theprovision of a stable alternating current arc. In order to stabilize analternating current arc for welding and the like, various suggestionshave been made. The addition of certain emissive agents to a consumableelectrode and/or to the arc aidsreignition at each zero crossover andhelps stabilize the alternating current are. Additionally, surge orpulse circuits have been added in the output circuit to provideadditional ignition power during the periods of reignition. Thus, highfrequency circuits have been suggested. These have certain disadvantagesfrom the standpoint of expense, complexity and regulation because of thehigh frequency signal radiation. Additionally, capacitance surge andsimilar discharge pulse circuits have been suggested wherein a strikingpulse is impressed in parallel across the arc in the vicinity of thecurrent at the time it is passing through zero; for example, as shown inUS. Patent 2,891,196.

In alternating current arc welding and the like, a relatively constantpotential power source rather than the constant current source isdesirable. Generally, it has been found necessary and desirable to shiftthe arc voltage with respect to the current such that a voltage isavailable to restrike the are as the current passes through zero.Normally, this will be done by the insertion of a suitable reactor orother inductance in series with the output leads. Although this has avery desirable result as far as restriking is concerned, it creates adrooping characteristic. The transformer output thus has acharacteristic somewhat similar to constant current units. This requiresa greater rated power transformer with the resulting ad- Patented June27, 1967 "ice ditional expense as well as the corresponding undesirablearc characteristics for certain applications.

The present invention is particularly directed to an are power supplyhaving a pulse or surge circuit incorporated as a part of the output ofthe main current circuit and producing an accurately timed reignitionpulse during the initial portion of each half cycle. The presentinvention is particularly directed to a relatively simple, inexpensiveand reliable circuit which can be employed with a constant potentialpower source to produce and maintain an exceedingly stable alternatingcurrent welding arc.

The present invention generally employs a pair of capacitive pulsecircuits connected in parallel with the power source, and in series withthe welding are, each of which includes a gate controlled or triggeredrectifier connected in series with a surge or pulse capacitor and ameans to trigger the rectifiers to provide controlled discharge of theenergy stored in the capacitors into the arc. The rectifiers areoppositely polarized such that one aids reignition during the positivehalf cycle and the other acts during the negative half cycle. Thecapacitors are sequentially and alternately charged by the alternatehalfcycles of a current to a relatively high voltage, generally four tofive times the normal arc voltage. When the controlled rectifier isfired, the corresponding capacitor injects a high voltage pulse into thecircuit, preferably during the beginning or initial portion of each halfcycle of the welding current. The controlled rectifiers carry only avery minimum amount of current for a very short time cycle andconsequently small and inexpensive controlled rectifiers can beemployed.

In a preferred construction, a surge transformer is provided having itsprimary connected across a portion of the main welding transformerprimary and having a pair of secondaries each of which is connected inparallel with a corresponding surge or pulse capacitor. Suitable diodesare connected in series with each of the surge secondaries and polarizedto oppositely charge the two capacitors and prevent discharge of thecapacitor through the secondary during the opposite half cycles.Additionally, a unijunction timing circuit is connected across thewelding power lines to periodically generate a firing pulse to thecorresponding rectifier. The unijunction timing unit includes a storagemeans charged from the main power lines in synchronism with the chargingof the corresponding capacitor. The timing is set to trigger thecontrolled rectifier at the immediately next succeeding half cycle andthus the capacitor is discharged in aid of the corresponding mainwelding half cycle. Generally, unijunction transistor timing circuitsemploying capacitor timing means have a tendency to discharge during theterminal portion of the timing half cycle which tends to fire therectifier just prior to the zero crossover point and therefore prior tothe initial half cycle in which it is to provide an aiding voltagepulse. In accordance with the present invention, a short additionaltiming delay means is inserted in the timing circuit to delay the firingof the controlled rectifier junction and thereby insure firing duringthe initial portion of the next half cycle. Although it may not occurprecisely at the zero crossover point, the pulse will always be earlyenough to provide stability without possible complete loss which mayoccur if exact zero crossover synchronism were attempted.

The drawing furnished herewith illustrates the above advantages andfeatures of the present invention as well as others which will be clearto those skilled in the art.

In the drawing:

FIG. 1 is a schematic circuit diagram of an alternating current constantpotential arc welding source constructed in accordance with the presentinvention; and

FIG. 2 is a voltage and current versus time trace showing the mainwelding voltage and current, the surge charging current and the highvoltage pulses.

Referring to the drawing and particularly to FIG. 1, a single phase arcwelding system is shown including a single phase, constant potential arcwelding transformer 1 connected to a set of incoming power lines 2. Theoutput of the transformer 1 is connected to a work member 3 and to anelectrode nozzle 4 through which a consumable electrode 5 is fed towardthe work 3. The transformer 1 provides power across the work member 3and the electrode 5 to establish and maintain an alternating currentwelding are 6. Electrode drive means 7 are coupled to the electrode 5 tocontinuously feed the consumable electrode 5 to the arc 6.

The electrode feed and current transfer mechanism is showndiagrammatically as many different designs might be used which will beclear to those in the welding art.

The voltage impressed across and the current through arc 6 is analternating current having alternate positive and negative half cycles.At the crossover point in the arc current, the arc 6 must actually bereignited with current flow in the opposite direction. In accordancewith the illustrated embodiment of the invention, pulse or surge sources8 and 9 are sequentially connected in series across the are 6 and areadapted as hereinafter described to periodically provide a high voltagepulse during the initial portion of each successive half cycle of themain welding current and voltage.

More particularly, the main welding transformer 1 includes a primarywinding 10 connected to the incoming power lines 2. A low voltagesecondary 11 is close coupled to the primary winding 10 through amagnetic core 12 in accordance with any well known or desirableconstruction and provides an essentially constant output voltage for allload or are current supplied to arc 6. A stabilizing reactor orinductance 13 is connected in series in the one load line between thesecondary winding 11 and the pulse sources 8 and 9 to assist in themaintaining and the stabilizing of the arc 6 and in particular toprevent complete extinction of the are 6 during the transition from halfcycle to the next half cycle. As in past practice, the reactor 13 servesto shift the voltage to precede the current and thereby maintainsvoltage available to strike the arc in the opposite direction as thecurrent passes through zero. As hereinafter described, in the presentinvention, the value of the reactor 13 can be substantially reduced as aresult of the interaction of the surge sources 8 and 9, although it hasbeen found that optimum results are obtained by maintaining somereactance in the circuit.

The surge sources 8 and 9 are similarly constructed and consequently thesource 8 will hereinafter be described in detail with correspondingelements in source 9 identified by similar primed numbers for simplicityand clarity of explanation.

The surge source 8 includes a silicon controlled rectifier 14 connectedin series with a surge capacitor 15 between the main welding current orload lines connected respectively to the work member 3 and the nozzle 4.A charging transformer 16 includes a primary winding 17 connected acrossone half of the main primary 10 of the welding transformer 1. Asecondary winding 18 is magnetically coupled to the primary winding 17,shown by a dashed coupling line, and connected in series with a diode 19and a current limiting resistor 20 across the related surge capacitor15. The diode 19 provides a polarized charging source such that thecapacitor 15 will be charged, as shown by the usual positive andnegative signs, during a selected half cycle of the alternating currentinput cycle. During the next half cycle, the diode 19 preventsdischarging of the capacitor 15 through the secondary or the transfer ofenergy from the secondary winding 18 to the capacitor. The capacitor 15therefore maintains its charged condition until such time as thecorresponding silicon controlled rectifier 14 is fired to discharge thecapacitor.

The silicon con-trolled rectifier 14 is a well known element including acathode 21 connected to the one output line and having an anode 22connected to the capacitor 15 which is connected to the opposite powerline. A gate 23 of the rectifier 14 can fire the silicon controlledrectifier into conduction whenever the anode 22 is positive relative tothe cathode 21 in accordance with well known operation of such devices.

The surge sources 8 also include a firing circuit connected to provide aproperly timed pulse to the gate 23 for firing the silicon controlledrectifier 14 and discharging of the capacitor 15 during the initialportion of a half cycle having a polarity at the secondary 11corresponding to the polarity of the charging capacitor 15. As a result,the discharge of the capacitor 15 provides an aiding voltage pulse tothe main welding transformer 1.

The illustrated firing circuit employs a well known unijunctiontransistor 24 having base electrodes 25 and 26 and a trigger or gateelectrode 27. A biasing resistor 28 is connected to the load line and tothe base electrode 25 with the junction 29 therebetween connected to thegate 23 of the silicon controlled rectifier 14. A bias resistor 30 inseries with a common resistor 31 is connected between the electrode 26and the opposite load line. A timing capacitor 32 in series with anadjustable timing resistor 33 is connected between the load line and theresistor 31 and thus in parallel with the unijunction transistor 24 andresistors 28 and 30. The trigger electrode 27 of the unijunctiontransistor 24 is connected to the junction 34 of capacitor 32 andresistor 33. A Zener diode 35 or the like is connected in parallel withthe timing capacitor 32 and the timing resistor 33 and is polarized toeffectively short circuit the timing circuit during one half cycle andto limit the voltage across the timing circuit during the alternate ortiming cycle. In operation, during the half cycle during which the diode35 is reverse biased, current flows through the timing resistor 33 andthe timing capacitor 32. At a selected charge level the trigger junction34 is at a level which causes the unijunction transistor 24 to conductbetween electrodes 25 and 27 whereupon the capacitor 32 rapidlydischarges through the trigger electrode 27, main electrode 25 and theresistor 28. This generates a firing pulse at the junction 29. This willturn on the silicon controlled rectifier 14 and discharge the capacitor15.

Generally, in such timing circuits, the unijunction transistor 24 willfire during the terminal portion of the charging half cycle of the mainwelding power. A holding capacitor 36 is connected in parallel with theunijunction transistor 24 and its resistors 28 and 30 and acts to delaythe firing, This prevents firing before the welding current has reachedzero.

The surge source 9 is basically identical to source 8 except that thecorresponding components are reversely connected with respect to thewelding power lines and consequently the silicon controlled rectifier14' is fired during the next half cycle. The capacitor 15' is chargedduring the half cycle that the capacitor 15 is discharged. During thenext half cycle, the voltage relationship of all of the transformersecondaries is reversed and capacitor 15' is discharged to aid thevoltage of the welding secondary and the capacitor 15 is again charged.

Referring to FIG. 2, voltage and current versus time traces are shown.For purposes of illustration, the main welding transformer secondary 11and surge secondaries 18 and 18 are shown with a properly relatedinstantaneous polarity at a voltage reversal or zero crossover by theconventional dots at the instantaneously positive end of thesecondaries. The typical curves of FIG. 2 are related to the aboveassumed polarities.

A voltage curve 36 is shown crossing the zero axis in a positivedirection at a selected reference time. The current curve 37 is asimilarly shaped sine curve which lags the voltage curve by a slightangle as a result of the inductance of reactor 13. Thus, when thecurrent curve 37 crosses the zero axis in the positive direction, thevoltage curve has risen to provide some restriking voltage.

The illustrated degree of phase shift between curves 36 and 37 isexaggerated for purposes of clarity. Ideally, the shift should beminimized to maintain the flat characteristic of a constant potentialmachine. Although a slight shift is desirable, the present inventionpermits substantial minimizing thereof without causing arc instability.The related output current of the secondaries 18 and 18' of the surgetransformer 16 is shown by the sinusoidal curve 38 which is shown out ofphase with the main transformer voltage by 180 degrees. The surgetransformer 16 is connected to provide this 180 shift with respect tothe main transformer 1. During the positive half cycle of the mainvoltage curve 36, the output of the surge transformer 16 provides anegative half cycle across the capacitors 15 and 15. In operation, thecapacitor 15' is charged as a result of the polarized connection ofdiode 19' and reaches a fully charged value in slightly more than onehalf of the period of the negative half cycle as shown by the crosshatched area under the corresponding half cycle of curve 39. Thepolarity will be as indicated.

The surgecapacitor 15 was fully charged by the prior half cycle to thepolarity indicated; that is, with the positive side connected throughrectifier 14 to the positive going line connected to the correspondingend of the transformer secondary 11.

The timing capacitor 32 and holding capacitor 36 were charged from themain welding transformer output by current from the power lead to theright of the drawing, 1esistor31, resistor 33 and capacitor 32 inparallel with capacitor 36 to the indicated polarity during the samehalf cycle that the main capacitor 15 was charged. At the terminalportion of the half cycle of the welding current trace 37, theunijunction transistor 24 would be biased to conduct by the voltage atjunction 34. The holding capacitor 36 however delays the firing voltagefor a short period until the main welding current curve 37 has reachedor'passed the zero current axis; at which time the voltage relationshipis such that the timing capacitor 32 rapidly discharges through thetransistor 24 and resistor 28 therebygenerating a firing signal atjunction 29 and firing the silicon controlled rectifier 14 to permit therapid discharge of the capacitor 15. Although the secondary 11 oftransformer 1 has its positive side then connected to the cathode ofrectifier 14, the capacitor 15 which is at a much higher voltageestablishes the anode 22 at a higher positive voltage such thatrectifier 14 fires when the trigger pulse is applied to gate 23.

' The discharge of capacitor 15 generates a high voltage pulse or spike,shown at 40 in FIG. 2, which aids the main transformer voltage.

During this portion of the cycle, the polarity of the secondary 18 andthe associated diode 19' is such that charging current flows to chargethe main storage capacitor 15 to the selected voltage and polarity whichoccurs during the cross hatched portion of the half cycle of curve 38.

During the half cycle that capacitor 15 was discharged, current alsoflows through the timing circuit for rectifier 14.-Thus, current flowsfrom the left line, through resistor 31 to the capacitor 36 in parallelwith the timing capacitor 32'. The timing period is set by adjustment ofresistor 33 and selection of capacitor 36 to fire transistor 24 andthereby rectifier 14 as the welding current of curve 37 goes in theopposite or negative direction in the illustration. The capacitor 15maintains the charged condition until the timing circuit during the nexthalf cycle is energized to trigger silicon controlled rectifier 14' anddischarge the capacitor 15'.

The surge sources 8 and 9 are thus alternately and sequentially chargedand discharged in timed relation with the corresponding half cycles ofthe welding current to produce sharp triggering pulses 40 during thefirst few degrees of the corresponding alternating power cycles. Formost applications, the pulses are established in the first five degreesof the half cycle of the welding current. The pulses preferably have apeak of 250 to 300 volts. However, capacitors provide an instantaneouscurrent surge generally in the order of 400 amperes for a period about50 microseconds. This instantaneous current surge provides suificientheat energy to insure arc reignition.

In accordance with the present invention, it has been found that thestabilizing reactor 13 can be made relatively small and further that theamount of emissive agents supplied to the arc 6 can be substantiallyreduced while maintaining a highly satisfactory and stable are which hasheretofore required a large reactor and/ or excessive amounts ofemissive agents. The reduction in required emissive agents is ofsubstantial significance as a practical matter in producing weldingelectrodes incorporating emissive agents. Where relatively substantialportions of emissive agents have been required in welding electrodes toprovide a highly satisfactory arc, the uniformity of the distribution ofthe emissive agents within the electrode was found to have a very markedeffect on the arc.

In practice, the arc may vary as much as 4; inch in length withelectrodes having a substantial amount of unevenly distributed emissiveagents or material. Although the arc will appear to be very stable, ithas been found that in fact the welds are not as satisfactory as wouldnormally be desirable, Thus, by reducing the magnitude of the emissiveagents required, the criticalness of the distribution is reduced and thecost of manufacturing electrodes is not only reduced by the amount ofreduced emissive agents employed but further by the additional costrequirements attendant providing accurate emissive agent distributionwithin the electrode.

The present invention thus provides a highly improved welding circuitwhich is of substantial significance in employing a combination ofdifferent elements for providing improved current sources. Thus, thepresent invention provides a new improved surge source which provides auniquely optimum operation when combined with an electrode having a lowamount of emissivity material therein and an inductance providing a veryminimal shift of the are supply voltage with respect to the current.Further, the circuit is a relatively simple and inexpensive circuitwhich can be readily mass produced and incorporated into aWelding'tran'sformer without unduly increasing the initial cost or thesubsequent maintenance and by employing solid state control elementswhich will have an exceptionally long, reliable life.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularlypointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. An arc current supply system for establishing and maintaining analternating current arc, comprising an arc power transformer to providean alternating current to an are at a selected voltage,

power lines connected to the transformer and adapted to be connected toare terminals for supplying alternating current to an arc,

a pair of surge stabilizing circuits each of which includes a pulsecapacitor in series with a solid state switching means to controldischarge of said pulse capacitor to the arc, said surge stabilizingcircuits each connected in series with the arc and in parallel with theare power transformer, each of said surge stabilizing circuits beingadapted to conduct a pulse of high voltage current to the arc during thebeginning period of each successive half cycle of alternating currentsupplied by said are power transformer to said are to facilitate arcreignition during welding,

means to charge said capacitors with opposite polarities, each capacitorbeing charged during the half cycle the opposite capacitor is beingdischarged at, and

timing means forming a part of the stabilizing circuits to successivelydischarge said capacitors through the arc to provide an instantaneoushigh voltage current pulse to stabilize the arc during the correspondinghalf cycle, said timing means including capacitive type firing means.

2. The are system of claim 1 having,

a reactance means serially connected in one of the power lines, and

means to supply an emissive material to the are.

3. An are system comprising,

a pulse single phase arc supply transformer having a primary and asecondary,

are power lines connected to the secondary and adapted to be connectedto are terminals for supplying single phase alternating current to anarc,

a pair of surge stabilizing circuits each of which includes a capacitorin series with a triggered switch ing means having a firing electrode,said surge stabilizing circuits being connected in parallel with the arcsupply transformer and in series with the arc during periods ofconduction,

trigger means connected to the firing electrode to discharge said pulsecapacitors through the are at the initial portion of each half cycle ofarc current, during arc welding to facilitate arc reiginition at thecommencement of each half cycle,

charging means having an input means connected to at least a portion ofthe primary and an output means connected to alternately andsequentially charge said pulse capacitors with opposite polarities, eachpulse capacitor being charged during the half cycle that the oppositepulse capacitor is being discharged through the arc.

4. An arc welding system comprising,

a single phase arc welding transformer having a primary and a secondary,

welding power lines connected to the secondary and adapted to beconnected to welding terminals for suppling alternating current to anare,

a pair of surge stabilizing circuits connected between the power linesand each of which includes a pulse capacitor in series with a switchingmeans, said switching means to control discharge of said pulse capacitorto the arc,

a reactor connected in series with a power line between the secondaryand the stabilizing circuits,

means to discharge said capacitors at the initial portion of the halfcycles of welding current to aid the corresponding half cycle, and

means connected in parallel with at least a portion of the primary tocharge said capacitors with opposite polarities, each capacitor beingcharged during the half cycle that the opposite capacitor is being discharged at.

5. A power system for establishing and maintaining an A.C. are,comprising to each of the said pulse capacitors to charge the respectivecapacitors successively during successive alternate half cycles, andcapacitor timing means connected to the output terminals and chargedthereby and connected to the silicon controlled rectifier to alternatelyfire said rectifiers and thereby enable sequential periodic discharge ofsaid pulse capacitors during the initiation of each successive halfcycle of welding current to assist arc reignition.

6. A power system for establishing and maintaining an A.C. arc,comprising a transformer having a primary defining power input terminalsand a secondary defining a pair of output terminal means,

a reactor connected in series with the secondary between the outputterminal means,

a pair of oppositely connected stabilizing circuits, each connected inparallel with said transformer and in series with the arc during theirrespective periods of conduction, and each including a siliconcontrolled rectifier having a gate, said rectifiers each being connectedin series with a pulse capacitor,

said rectifiers being oppositely polarized with respect to the terminalmeans,

charging means connected to the input terminals and to each pulsecapacitor to charge the respective pulse capacitor during successivealternate half cycles to a polarity to bias the corresponding siliconcontrolled rectifier to'conduct, when conditioned to conduct by therespective gate,

capacitor timing means connected to the output terminal means andconnected to the respective gates of said silicon controlled rectifiersto alternately fire the rectifiers in timed relation with the initialportion of the corresponding half cycle of the arc current.

7. An alternating current welding system comprising a power transformerfor supplying an alternating current to an are, said transformer havinga primary and a secondary,

a pair of welding lines including a welding electrode and a workpiececonnected in series with said secondary,

a pair of surge circuits each connected in parallel with saidtransformer and in series with the arc during periods of conduction,each surge circuit including a gate controlled rectifier connected inseries with a surge capacitor and in series with said electrodes, saidrectifiers being oppositely polarized to conduct during opposite halfcycles of the alternating current from said secondary, and each having acontrol gate to fire the rectifier into conduction,

a pair of related charging circuits connected one each to said surgecapacitors and each including a diode polarized to charge saidcapacitors to discharge through the related silicon controlledrectifiers,

a pair of rectifier firing circuits one for each of said rectifiers,each of the firing circuits comprising a triggered switch means tocontrol the related rectifier and a timing capacitor connected tocontrol the switch means to cause discharge of the respective pulsecapacitors through said electrode during successive half cycles of saidalternating current. 8. The system of claim 7 wherein each firingcircuit includes a unijunction transistor as the switch means connectedwith a timing capacitor in a timing circuit to the welding lines forcharging of said timing capacitor.

9. The system of claim 7 having a holding capacitor paralleled with theuniiunction transistor.

10. An alternating current welding system, comprising a powertransformer supplying an alternating current to an are, said transformerhaving a primary, a secondary,

and main power lines, a welding circuit serially con:

nected by said main power lines to the secondary of said powertransformer, said welding circuit including an electrode and aworkpiece,

a pair of surge circuits each connected in parallel with said powertransformer and in series With said electrode during periods ofconduction, said surge circuits each including a silicon controlledrectifier connected in series with a surge capacitor and said electrode,said rectifiers being oppositely polarized to conduct during oppositehalf cycles of the alternating current from said secondary, and therebycause said surge capacitors to discharge during successive half cyclesof said alternating current, each rectifier having a control gate tofire the rectifier into conduction,

a surge transformer having a single primary connected across one half ofthe primary of the power transformer and having a pair of secondaries,said secondaries each being connected in parallel with the correspondingsurge capacitor, and in series with the corresponding rectifier,

a pair of alternately and sequentially operable charging circuits onefor each of said surge capacitors, and

a pair of rectifier firing circuits one for each of said rectifiers,each of the firing circuits comprising a unijunction transistor havingmain power terminals connected between the electrode and the workpiece,said firing circuits each including biasing resistors and a timingcapacitor connected in series with an adjustable resistor across themain power lines with the unijunction transistor having a triggerterminal connected to the junction of the capacitor and the adjustableresistor, each of said firing circuits further including a holdingcapacitor connected in parallel with the unijunction transistor to delaythe firing of said unijunction transistor so that the surge capacitorsdischarge through said are sequentially during the initial portion ofthe successive half cycles of welding current to aid in arc reignitioneach half cycle and thereby stabilize the arc.

11. An alternating current welding system, comprising an alternatingcurrent power transformer having a power primary and a power secondary,

an electrode and a workpiece serially connected to the secondary of saidpower transformer,

a pair of surge circuits each including a silicon controlled rectifierconnected in series with a surge capacitor across the welding lines,said rectifiers being oppositely polarized to conduct during oppositehalf cycles of the alternating current from said secondary, and eachhaving a control gate to fire the rectifier into conduction,

a reactor connected in series between the secondary and the surgecircuits,

a surge transformer having a single surge primary connected across onehalf of the power primary of the power transformer and having a pair ofsurge secondaries,

a pair of related charging circuits connected one each in parallel withits associated surge capacitor, each charging circuit including one ofsaid secondaries of the surge transformer connected in series with adiode and a current limiting resistor, each diode being polarized tocharge its associated capacitor to discharge through the related siliconcontrolled rectifiers, and through the electrode to said workpiece, and

a pair of rectifier firing circuits one for each of said rectifiers,.each of the firing circuits comprising a unijunction transistor having afiring terminal and having main power terminals connected across thewelding lines with biasing resistors, said circuits each including atiming capacitor connected in series with an adjustable resistor acrossthe main power terminals of the unijunction transistor and having thetrigger terminal connected to the junction of the capacitor and theadjustable resistor, each of said firing circuits further including aholding capacitor connected in parallel with the unijunction transistorto delay the firing of said unijunction transistor whereby said surgecapacitors are sequentially first charged and then subsequentlydischarged through the electrode to said workpiece during successivehalf cycles of alternating current flow.

References Cited UNITED STATES PATENTS 2,558,102 6/1951 Roberts 315-2892,880,376 3/1959 Tajbl et a1. 315-289 3,249,799 5/1966 Powell 315-983,264,451 8/ 1966 Stauverman 315-100 FOREIGN PATENTS 823,210 4/ 1960Great Britain.

JOHN W. HUCKERT, Primary Examiner. D. Q. KRAFT, Assistant Examiner,

1. AN ARC CURRENT SUPPLY SYSTEM FOR ESTABLISHING AND MAINTAINING ANALTERNATING CURRENT ARC, COMPRISING AN ARC POWER TRANSFORMER TO PROVIDEAN ALTERNATING CURRENT TO AN ARC AT A SELECTED VOLTAGE, POWER LINESCONNECTED TO THE TRANSFORMER AND ADAPTED TO BE CONNECTED TO ARCTERMINALS FOR SUPPLYING ALTERNATING CURRENT TO AN ARC, A PAIR OF SURGESTABILIZING CIRCUITS EACH OF WHICH INCLUDES A PULSE CAPACITOR IN SERIESWITH A SOLID STATE SWITCHING MEANS TO CONTROL DISCHARGE OF SAID PULSECAPACITOR TO THE ARC, SAID SURGE STABILIZING CIRCUITS EACH CONNECTED INSERIES WITH THE ARC AND IN PARALLEL WITH THE ARC POWER TRANSFORMER, EACHOF SAID SURGE STABILIZING CIRCUITS BEING ADAPTED TO CONDUCT A PULSE OFHIGH VOLTAGE CURRENT TO THE ARC DURING THE BEGINNING PERIOD OF EACHSUCCESSIVE HALF CYCLE OF ALTERNATING CURRENT SUPPLIED BY SAID ARC POWERTRANSFORMER TO SAID ARC TO FACILITATE ARC REIGNITION DURING WELDING,MEANS TO CHARGE SAID CAPACITORS WITH OPPOSITE POLARITIES, EACH CAPACITORBEING CHARGED DURING THE HALF CYCLE THE OPPOSITE CAPACITOR IS BEINGDISCHARGED AT, AND